Protocol Buffers are a super efficient and very flexible way of serializing structured data. Developed at Google, provide the developers lightspeed serialization and deserialization capabilities.
There a handful of .NET implementations, the most popular one, called protobuf-net, created by Marc Gravell from StackExchange. On top of that library, WebApiContrib project has a Web API Protocol Buffers formatter ready for you to plug in.
More after the jump.